A lubricating oil is usually blended with an antioxidant for the purposes of improving shelf life or product stability. For the antioxidant, phenol based, amine based, sulfur based and phosphorus based types of antioxidants exist. However, in view of problems such as waste oil treatment, as there is a tendency to avoid additives containing sulfur or phosphorus atoms in lubricating oils, phenol or amine based antioxidants are usually used. In addition, due to the increase in high performance of machinery requiring lubricating oil, there has been an increase in demand for high temperature lubricating oils and a reduction of the amount of waste oil treated which has led to the need for lubricating oils with even longer shelf lives, thus the marketplace has placed high demands on improved performance antioxidants.
In view of this, in order to improve the anti-oxidation performance of lubricating oils, a method capable of realizing an increase in the shelf life of lubricating oils by blending ratios of various types of antioxidants has been proposed. For example, Patent Document 1 discloses a long life fuel saving engine oil composition characterized in that a mineral oil and/or a synthetic base oil contains an amine-based antioxidant and a phenol-based antioxidant at 1.2 mass % or more in total, in which a mass ratio (N/O) of a nitrogen content (N) of the amine-based antioxidant to an oxygen content (O) of the phenol-based antioxidant is 0.06 to 0.5, and contains zinc dialkyldithiophosphate (ZnDTP) at 0.02 to 0.06 mass % in terms of phosphorus (P), and further contains molybdenum dithiocarbamate (MoDTC) at 0.045 mass % or more in terms of molybdenum (Mo). Further, Patent Document 2 discloses a lubricant composition containing a natural and/or synthetic lubricating base oil, a first antioxidant formed of a particular secondary diarylamine, a second antioxidant formed of a particular substituted p-phenylenediamine, and a third antioxidant formed of a particular substituted phenol.
Further, a method involving the use of an antioxidant containing a metal such as molybdenum has also been proposed. For example, Patent Document 3 discloses an antioxidant composition characterized by it containing an organic molybdenum compound and a lubricating oil base oil having a total aromatic content of 20 to 100 mass %. In addition, a method involving the use of a hindered amine-based antioxidant has also been proposed. For example, Patent Document 4 discloses an antioxidant composition for a lubricating oil, characterized by containing a particular amine compound and a phenol compound containing one or more ester groups.